You want Mullards. When I owned the Calypso I found them to be very smooth.
Mullard 10's will cost you a fortune. Read the threads by Perfectionist. He recommends a few since he did a great deal of tube rolling. Also the great Jadem6 wrote a fabulous review and also identified so good tube to use with this pre-amp.
I just got mine and have Siemens 6922's and Dynaco 12ax7's. However, I expect to do some tube rolling. Why? Because I still have my Audio research LS 25 MKl and this per-amp is giving this new Calypso a real RUN FOR THE MONEY!!!!
I have replaced the standard 6922 Reflector tubes with a Russian Reflector 6H23P-EB, witch results in greater dynamics and swing, But a bit more edgy style USD 22.5 at Parts connexion.
I have also replaced the standard 12ax7 with a new production of Tungsol ECC803S Gold Pin, witch results in deeper soundstage. USD 29.95 at Parts connexion.
I am still seeking for a more lucid and full born sound.
I am currently using the Calypso. I have done some tube swapping, and have currently settled on the Mullard 10M 12ax7 gold pin and the Amperex 6922 PQ white label. I have found this combination very musical, although I admit to being influenced by all of the 'tube lore' floating around out there. Unfortunatly, these tubes are hard to come by. I was using the Mullard M8137 (for the 12ax7 slot) prior to the 10M, and I must say it was also excellent. Those are easier to come by, and much less expensive. I haven't AB'd them in a while, but I don't believe there is a huge difference. Also, long plate Mullards should be good.
Of course, it is a matter of personal taste, system synergy, and the quality of the actual tubes that will ultimately tell which is best for your system.
I had an opportunity to try several different nos brands, and ultimately decided to go with Ediswan 6922's and Telefunken 12AX7's. The Ediswan's are really smooth, yet detailed, and presented somewhat of a sweeter signature in the upper and middle registers. The bass was also very good and tuneful. The Eddies coupled with the Telefunken proved to be a great combo in the Calypso. Thank you for all your help.